Various shoes such as sports shoes are provided with soles.
The soles can be roughly divided into a non-foamed sole made of a non-foamed molding formed into a prescribed shape and a foamed sole made of a foam molding formed into a prescribed shape.
A shoe provided with the foamed sole is lighter in weight than a shoe provided with the non-foamed sole, and is excellent in wearing impression.
The shoe provided with a foamed sole is produced by adhering a foamed sole formed into a prescribed shape to the lower surface of a shoe body.
For example, the foamed sole is adhered to a shoe body with a solvent-type adhesive. In this case, in order to solidify the solvent-type adhesive, a hot air drying treatment is performed at a prescribed temperature (e.g. 60° C. to 80° C.). The foamed sole is heated, and then, the foamed sole shrinks and the dimensions of the foamed sole vary. Such a foamed sole in which a dimensional change is generated has a problem that the foamed sole cannot adhere to a prescribed position of the shoe body with precision.
In particular, the more the expansion ratio of a foam molding is increased (that is, the more the density of a foam molding is reduced), the more the rubber component is greatly stretched on molding, and therefore, thermal shrinkage of the foamed sole is liable to be generated at the time of the heat treatment. On the other hand, although the thermal shrinkage is hardly generated when the expansion ratio of a foam molding is reduced, such a foamed sole is relatively heavy in weight and is poor in wearing impression.
Patent Document 1 discloses a foamable rubber composition for a shoe sole prepared by blending a polymer component composed of 10 to 90% by mass of a vinyl-cis-polybutadiene rubber (A) containing 1,4-cis-polybutadiene (a) and a 1,2-polybutadiene crystalline fiber (b) having a prescribed shape and a melting point of 170° C. or higher, 10 to 50% by mass of a diene-based rubber (B) other than the rubber (A), and 2 to 50% by mass of a thermoplastic polymer (C); and 2 to 50 parts by mass of a rubber-reinforcing agent (D) relative to 100 parts by mass of the polymer component.
Patent Document 1 discloses various effects such as “the rubber composition for a shoe sole is light in weight and has a moderate hardness, is excellent in tensile strength, tear strength, abrasion resistance, and gripping property, and is also excellent in dimensional stability after foaming.”
However, from the description in Patent Document 1, it is not clear what kind of action is exerted to obtain a foam molding excellent in dimensional stability after foaming in the case of using the rubber composition for a shoe sole.
Moreover, Patent Document 1 does not disclose nor suggest about thermal shrinkage of a foam molding.
[Patent Document]
[Patent Document 1] WO 2008/013060
A first object of the present invention is to provide a foamed sole which hardly shrinks when it is heated and a shoe prepared therewith.
A second object of the present invention is to provide a foamed sole which is relatively light in weight and hardly undergoes thermal shrinkage and a shoe prepared therewith.
According to an assumption made by the present inventors, the cause of the thermal shrinkage of a foamed sole is considered to be as follows. In the present specification, “thermal shrinkage” means shrinking when heat is applied, and differs from the phenomenon of gradually shrinking spontaneously under ordinary temperature condition.
With regard to a foam molding containing a rubber component, the more it is allowed to highly foam, the more the rubber component is greatly stretched, and therefore, the force of going to return to the original state (shrinkage) is generated strongly. As such, a foam molding obtained by allowing a forming material containing the rubber component to highly foam tends to easily undergo thermal shrinkage. In addition, it is considered that in the case where a resin component is added to the forming material for a foam molding, thermal shrinkage thereof can be suppressed since the resin component resists shrinkage caused by the rubber component. In this connection, being allowed to highly foam mentioned above means being high in expansion ratio based on general foamed soles.
However, in the case where a forming material containing a rubber component and a resin component is allowed to highly foam, the resin component, as well as the rubber component, is greatly stretched. As such, in the case where the foam molding is heated, the rubber shrinkage inhibitory action which the resin component has is diminished since the stretched resin component also shrinks.
Based on such an assumption, the present inventors have obtained the findings that by using a forming material in which the stretched resin component and rubber component hardly shrink when exposed to a temperature within a prescribed range, a foamed sole which hardly undergoes thermal shrinkage can be constituted, and thus, the present invention has been completed.
The foamed sole according to the present invention includes a foam molding containing a rubber component and a resin component, the maximum value of a loss factor [tan δ] at a frequency of 10 Hz and at 30° C. to 80° C. of the foam molding is 0.18 or less, and a peak of a loss factor [tan δ] at a frequency of 10 Hz of the foam molding lies within the range of 100° C. or higher.
In the production of a general shoe, as described above, although a foamed sole is heated in the adhesion process, the temperature lies within the range of 60° C. to 80° C. Moreover, even when the shoe are exposed to a high temperature under a wearing environment of the shoe, the temperature lies within the range of 30° C. to 40° C. Therefore, a foamed sole which hardly undergoes thermal shrinkage in a temperature range of 30° C. to 80° C. is satisfactory in either of production and wear.
In this regard, the foamed sole according to the present invention is made of a foam molding in which the maximum value of a loss factor [tan δ] at a frequency of 10 Hz and at 30° C. to 80° C. is 0.18 or less.
The smaller the loss factor (loss factor=loss elastic modulus/storage elastic modulus) is, the smaller the loss elastic modulus with respect to the storage elastic modulus. Thus, with regard to the foam molding according to the present invention which has a small loss factor at 30° C. to 80° C., shrinkage caused by the stretched resin component and rubber component can be suppressed since the material itself hardly softens in the temperature range. With regard to the foamed sole according to the present invention which is made of the foam molding, thermal shrinkage, which is a problem caused under a wearing environment of the shoe or on production thereof, can be suppressed.
Moreover, the foamed sole according to the present invention is made of a foam molding in which a peak of the loss factor [tan δ] lies within the range of 100° C. or higher.
With regard to such a foam molding in which a peak of the loss factor lies within the range of 100° C. or higher, the deformation amount is large at the foam molding temperature (e.g. 150° C. to 170° C.), and it is possible to allow the foam molding to relatively largely foam. On the other hand, the foam molding, as described above, hardly undergoes thermal shrinkage after a foam molding treatment. Although the weight reduction and the thermal shrinkage inhibitory action which are in a trade-off relationship have hitherto been deemed to be common general technical knowledge of a person skilled in the art, according to the present invention, it is possible to provide a foamed sole which is relatively light in weight (is allowed to relatively highly foam) and hardly undergoes thermal shrinkage. For example, according to the present invention, it is also possible to provide a foamed sole made of a foam molding with a density of 0.6 g/cm3 or lower.
In a preferred foamed sole according to the present invention, the foam molding contains a resin in which the lowest temperature among a glass transition point, a melting point, and a softening point of the resin is 90° C. or higher as the resin component.
In a further preferred foamed sole according to the present invention, the foam molding does not contain a resin in which the lowest temperature among a glass transition point, a melting point, and a softening point of the resin is lower than 75° C. as the resin component.
In a further preferred foamed sole according to the present invention, the loss factor [tan δ] at a frequency of 10 Hz and at 30° C. to 80° C. of the foam molding is 0.16 or less.
In a further preferred foamed sole according to the present invention, the loss factor [tan δ] at a frequency of 10 Hz and at 30° C. to 80° C. of the foam molding is 0.12 or more.
According to another aspect of the present invention, a shoe is provided.
The shoe according to the present invention is provided with any one of the foregoing foamed soles.
The foamed sole according to the present invention has a feature that it hardly shrinks when it is heated. Since the foamed sole hardly undergoes a dimensional change, for example, when it is adhered to a shoe body, it is possible to adhere the foamed sole to a prescribed position of the shoe body with precision.
Furthermore, according to the present invention, it is possible to provide a foamed sole which is relatively light in weight and hardly undergoes thermal shrinkage.
Moreover, the shoe according to the present invention hardly undergoes deformation even when exposed to a high temperature during wear, and is excellent in durability.